TwinDB

Include: TwinDB.h

Summary

TwinDB is an embedded key/value database based on BerkeleyDB. It basically adds security from database corruption on top of the non-transactional version of BerkeleyDB. I wrote it because the transactional version of BerkeleyDB is expensive ($100,000 for a commercial license).

It accomplishes this by maintaining twin BerkeleyDB databases and using "lock" files to indicate when a set of writes are in progress.

Put and delete entries are cached in memory until TwinDB_sync() is called or a read occurs (by calling TwinDB_get()). Then, it:

1. locks the first database
2. writes to it
3. unlocks it

and repeats this process for the backup database. Upon startup, it checks to see if a lock is on either file. If so, it assumes that database file is corrupt and replaces it with the other.

Cons:

• writes are 1/2 as fast
• database is 2x as big

Pros:

• the database cannot be corrupted
• it's still *very* fast
• it's free for commercial use
• the size difference might not matter (current hard disks are typically over 100G, so a 500M database will still only take up less than 1% of the drive if doubled)

Interface

TwinDB *TwinDB_new(void)

Creates an instance of TwinDB.


TwinDB_free(TwinDB *self)

Frees the specified instance of TwinDB, closing it if needed.


void TwinDB_path_(TwinDB *self, char *path);

Sets the file path of the database. The instance will create two files at the specified path with the suffixes "1.tdb" and "2.tdb".


void TwinDB_setWriteCacheSizeLimit_(TwinDB *self, size_t size)

Sets the limit of the write cache in bytes. Onces the write cache exceeds this limit, TwinDB_sync() will automatically be called. If the size is set to zero (which is the default), there is no limit.


void TwinDB_open(TwinDB *self);

Open the database. TwinDB_path_() must be called first.


void TwinDB_close(TwinDB *self);

Close the databases.


void TwinDB_delete(TwinDB *self);

Delete the databases.


void TwinDB_sync(TwinDB *self);

Save any cached writes (put or delete operations).


void TwinDB_put(TwinDB *self, DBT *key, DBT *value);

Set a key value pair. This operation is cached in memory until TwinDB_sync() or TwinDB_read() is called.


void TwinDB_get(TwinDB *self, DBT *key, DBT *value);

Get the value for the specified key.


void TwinDB_del(TwinDB *self, DBT *key);

Delete the specified key if present. This operation is cached in memory until TwinDB_sync() or TwinDB_read() is called.